Fixed broken class instance check in FunctionToInstanceMethodPass.

Fixes https://github.com/mono/CppSharp/issues/1081.

src/Generator/Passes/FunctionToInstanceMethodPass.cs

@@ -18,6 +18,9 @@ namespace CppSharp.Passes
 
         public override bool VisitFunctionDecl(Function function)
         {
+            if (AlreadyVisited(function))
+                return false;
+
             if (!function.IsGenerated)
                 return false;
 
@@ -79,7 +82,7 @@ namespace CppSharp.Passes
             if (classParam.Type.IsPointerTo(out tag))
             {
                 @class = tag.Declaration as Class;
-                return true;
+                return @class != null;
             }
 
             return classParam.Type.TryGetClass(out @class);

tests/Native/Passes.h

@@ -77,3 +77,10 @@ class ClassWithAbstractOperator
 {
   virtual operator int() = 0;
 };
+
+enum ConnectionRole
+{
+    Role1, Role2
+};
+
+bool ConnectionRoleToString(const ConnectionRole& role, const char* role_str);